Compliance with the specified tablet weight is an important quality parameter for tabletting. Any automatic control of a tabletting press is based on the relationship between the measured maximum pressing force of a tablet and the mass of the material filled into the die (tablet weight), assuming identical material data (bulk density). There is a direct relationship between the tablet weight and the pressing force needed to manufacture the tablet checked. This relationship is used for automatic monitoring and control devices on rotary presses. Depending on the material to be tabletted, a defined pressing force is associated with each tablet weight at a tablet shape preset by the compression molding dies and at a set tablet height. If the amount of the material filled in and consequently the tablet weight vary at constant tablet height, this results in a change in the pressing force in a direct dependence.
The parameters weight, height and hardness can be set more or less directly by means of specific operating parameters for every individual compression mold on modern rotary tabletting presses which have many individual compression molds on a circular path and press a large number of tablets per second.
However, the ratio of the specific operating parameters of the individual compression mold to the tablet parameter reached must be checked and regulated continuously because of the variety of the properties of the masses to be tabletted. The unambiguous assignment of the tablet parameters obtained in reality to the operating parameters of the individual compression mold must be ensured.
Devices have already been known, with which individual checks of the parameters weight, height and hardness can be performed on one tablet.
In Pharmacheck 1 (Operating Instructions for Pharmakontroll 3 NET, Chapter 3, pp. 11, 16-18, of Korsch Pressen GmbH, 1992/1993), individual tablets are sorted out from the tabletting press and are weighed in a scale. After weighing the first tablet, the scale is set to zero and a second tablet is weighed. When there are 10 or 20 tablets on the weighing scale, the scale is emptied, and the tablets are removed according to quality via three different outlet channels. Instead of the individual weighing, a batch of 10 or 20 tablets may be removed simultaneously from the tabletting press and weighed together. The weight of the individual tablets is determined by the weighings to an accuracy of 0.1 mg. The result of the weighing is used to control the degree of filling of the individual dies.
In Pharmacheck 2 (Operating Instructions for Pharmakontroll 3 NET, Chapter 4, pp. 5, 9, 10, 12-16, of Korsch Pressen GmbH, 1992/1993), individual tablets are fed to the spider rotatable around a vertical axis, and the individual tablets are fed by a rotation of the spider one after another to a weighing unit for determining the weight, to a measuring unit for determining the height or thickness, and to another measuring unit for determining the diameter and the hardness, and subsequently to an ejector unit. The process is timed at a rate of at least 6 sec per tablet and per station, so that only a small number of tablets can be checked in the case of a tabletting press having a working speed of 10,000 tablets per minute.
In addition, it is a drawback of the prior-art solutions that the performance of individual checks does not make possible a productive measurement process on a plurality of tablets, and the process cannot be carried out accurately based on individually determined tablet parameters, especially the tablet weight. As a result, the percentage of rejects is unsatisfactorily high during tabletting. It is practically impossible to resort to a certain set of data, which would make it possible, in the case of the weighing of individual tablets, to reliably recognize that the weight of the individual tablet determined corresponds to that of all the tablets produced. The parameters for height and hardness are to be determined only thereafter.
The determination of the weight, height and hardness on one tablet one after another cannot be assigned to the compression mold producing the tablet being checked with sufficient reliability, so that corrections performed on the tabletting press upon the detection of deviations often remain ineffective.
A considerable cleaning must be performed on the checking device in the case of a changeover from one type to another, because the dust-like abraded product is distributed in the entire device during the pressing and conveying of the tablets. The checking results are sometimes distorted as a result.
DE-GM 94 06 712 discloses a device for deburring and dedusting tablets with suction air, in which the tablets are conveyed by the suction of the exhaust unit over a plate chute between the tablet feed station and the tablet discharge station and are deburred and dedusted in the process. This solution is unsuitable for the subsequent checking steps on the tablets.
To carry out the height and hardness measurements, the tablets must be placed in the checking device such that tablets with approximately equal dimensions for thickness, length and diameter can also be measured in a reliable manner. The prior-art devices are unsatisfactory for this and prevent corresponding incorrect measurements only insufficiently.